Method and apparatus for cell reselection

ABSTRACT

A method and wireless transmit receive unit (WTRU) are disclosed that is configured to perform cell reselection to another cell when the WTRU is in a CELL_FACH state using an Enhanced-Dedicated Channel (E-DCH). The cell reselection is based on internal measurements by the WTRU. Alternatively, the cell reselection can be WTRU based on the WTRU measurements reported to the network.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/875,059 filed Oct. 5, 2015, which is a continuation of U.S. patentapplication Ser. No. 13/665,734 filed Oct. 31, 2012, which issued asU.S. Pat. No. 9,161,285 on Oct. 13, 2015, which is a continuation ofU.S. patent application no. 12/346,631 filed Dec. 30, 2008; which issuedas U.S. Pat. No. 8,325,684 on Dec. 4, 2012, which claims the benefit ofU.S. provisional Application Ser. No. 61/018,567, filed on Jan. 2, 2008,which are incorporated by reference as if fully set forth.

BACKGROUND

Wireless communication systems are well known in the art. Communicationsstandards are developed in order to provide global connectivity forwireless systems and to achieve performance goals in terms of, forexample, throughput, latency and coverage. One current standard inwidespread use, called Universal Mobile Telecommunications Systems(UMTS), was developed as part of Third Generation (3G) Radio Systems,and is maintained by the Third Generation Partnership Project (3GPP).

An example UMTS system architecture in accordance with current 3GPPspecifications is depicted in FIG. 1. The UMTS network architectureincludes a Core Network (CN) interconnected with a UMTS TerrestrialRadio Access Network (UTRAN) via an Iu interface. The UTRAN isconfigured to provide wireless telecommunication services to usersthrough wireless transmit receive units (WTRUs), referred to as userequipments (UEs) in the 3GPP standard, via a Uu radio interface. Acommonly employed air interface defined in the UMTS standard is widebandcode division multiple access (W-CDMA). The UTRAN has one or more radionetwork controllers (RNCs) and base stations, referred to as Node Bs by3GPP, which collectively provide for the geographic coverage forwireless communications with WTRUs. One or more Node Bs is connected toeach RNC via an Iub interface; RNCs within a UTRAN communicate via anIur interface.

WTRUs in a UMTS Terrestrial Radio Access Network (UTRAN) can be ineither of two modes: Idle or Connected. Based on WTRU mobility andactivity while in connected mode, the UTRAN can direct the WTRU totransition between a number of sub-states, e.g., CELL_PCH, URA_PCH,CELL_FACH, and CELL_DCH. User Plane communication between the WTRU andthe UTRAN is only permitted while in CELL FACH and CELL DCH state. TheCell DCH state is characterized by dedicated channels (DCHs) in both theuplink (UL) and the downlink (DL). On the WTRU side, this corresponds tocontinuous transmission and reception and can be demanding on user powerrequirements. The CELL_FACH state does not use DCHs and thus allowsbetter power consumption, at the expense of a lower uplink and downlinkthroughput.

The CELL_FACH is well-suited for signaling traffic (for example, thetransmission of CELL/URA UPDATE messages), and for applicationsrequiring very low uplink throughput. In CELL_FACH, uplink communicationis achieved through a random access transport channel (RACH) mapped to apacket random access channel (PRACH) physical channel. The RACH is acontention based protocol with a power ramp-up procedure to acquire thechannel and to adjust transmit power.

Downlink communication is through a shared Forward Access TransportChannel (FACH) mapped to a secondary common control physical channel(S-CCPCH) or through the high speed downlink channel.

Mobility is handled autonomously by the WTRU in CELL_FACH. The currentlysoft handover does not (as of Release 6 of the standard) exist withinCELL_FACH. As such, the WTRU independently takes measurements, anddetermines when to make cell reselections.

System information during CELL_FACH is read from a broadcast channel(BCH). This information includes the setup details for the uplink RACH,the downlink FACH and the high speed downlink shared channel (HS-DSCH))channels to be used in CELL_FACH.

Recent work by the standardization bodies has identified reuse of anEnhanced-DCH (E-DCH) in the CELL_FACH state. Enhanced-DCH is a featurethat was introduced to increase uplink throughput. The E-DCH operates ona request/grant principle. WTRUs send an indication of the requestedcapacity they require through a combination of mechanisms, while thenetwork responds with grants to these requests. These grants aregenerally generated by a Node B scheduler.

At the same time, Hybrid Automatic Repeat Requests (HARQs) are used inconnection with the physical layer transmissions. To facilitate theabove mechanisms, two new UL physical channels have been introduced, anEnhanced-Dedicated Physical Control Channel (E-DPCCH) for control, andan Enhanced-Dedicated Physical Data Channel (E-DPDCH) for data. Threenew downlink (DL) physical channels, two for transmission of grants andone for fast physical layer acknowledgements (Layer 1 ACK/NACK), werealso introduced. The Node B, therefore, is permitted to issue bothabsolute grants and relative grants. Grants are signaled in terms of apower ratio. Each WTRU maintains a serving grant, which it can convertto a payload size. For Release 6 WTRUs, mobility is handled by thenetwork through soft handover and active sets.

In addition to the new channels at the physical layer, E-DCH is alsorequired at the Medium Access Control (MAC) layer, with the introductionof new MAC-e/es protocol entities to handle the Enhanced DedicatedTransport Channel (E-DCH).

One of the concerns with the use of E-DCH in CELL_FACH is theinteraction of the uplink procedure with the mobility procedure, inparticular, the cell reselection procedure. This procedure can eitherremain WTRU autonomous or could be network assisted in some way. In bothcases, the network and WTRU actions upon a cell reselection need to bedefined. On the WTRU side, actions have to be specified to deal with themedium access control entities (MAC-e/es), hybrid automatic repeatrequest (HARQ) buffers, MAC Transmission Sequence Numbers (TSN), and thelike. With respect to the network, a serving radio network controller(SRNC) may need to be made aware when a new enhanced radio networktemporary identifier (E-RNTI) has been assigned by a controlling radionetwork controller (CRNC). The network may also have to deal withreleasing the resources in the source cell.

Accordingly, there exists a need for a method and apparatus to addressreselection for WTRUs capable of using the E-DCH while in Cell_FACHstate.

SUMMARY

A method and wireless transmit receive unit (WTRU) are disclosed that isconfigured to perform cell reselection to another cell. When the WTRU isin a CELL_FACH state using an Enhanced-Dedicated Channel (E-DCH). Thecell reselection can be network assisted based on WTRU measurementsreported to the network. Alternatively, the cell reselection can be WTRUbased.

BRIEF DESCRIPTION OF THE DRAWING

A more detailed understanding of the method may be had from thefollowing description, given by way of example and to be understood inconjunction with the accompanying drawing.

FIG. 1 is a block diagram of an overview of the system architecture of aconventional UMTS network;

FIG. 2 is a block diagram of a wireless communication system; and

FIG. 3 is a timing diagram illustrating a CELL_UPDATE procedure with aMAC-es included in the controlling radio network controller (CRNC).

DETAILED DESCRIPTION

When referred to hereafter, the terminology “wireless transmit/receiveunit” (“WTRU”) includes but is not limited to a user equipment (UE), amobile station, a fixed or mobile subscriber unit, a pager, a cellulartelephone, a personal digital assistant (PDA), a computer, or any othertype of user device capable of operating in a wireless environment. Whenreferred to hereafter, the terminology “base station” includes but isnot limited to a Node B, a site controller, an access point (AP), or anyother type of interfacing device capable of operating in a wirelessenvironment. When referred to hereafter, the terminology “E-DCH systeminformation” includes but is not limited to E_DCH information that isbroadcast by the Node B as part of its system information. This mayinclude information for a set of shared E-DCH resources that areassigned by the NodeB and shared by WTRUs in CELL_FACH state. Theterminology enhanced random access channel (E-RACH) refers to theenhanced uplink mechanism used in CELL_FACH state, including, but notlimited to the use of Enhanced-DCH resources.

A Medium Access Control (MAC) layer is divided into several entities;MAC-e/es protocol entities are preferably provided to handle an EnhancedDedicated Transport Channel (E-DCH). Generally, the expression “MAC-is”,“MAC-i”, and “MAC-is/i” may be substituted for “MAC-es”, “MAC-e” and“MAC-es/e” respectively. “MAC-es/e reset” procedure may be replaced by“MAC-is/i reset” procedure where the “MAC-is/i reset” procedure refersto a procedure similar to “MAC-es/e reset” with the possible additionalstep that segmentation buffers are flushed.

Referring to FIG. 2, a HSPA wireless communication network (NW) 10comprises a WTRU 20, one or more Node Bs 30, and one or more cells 40.Each cell 40 comprises one or more Node Bs (NB or eNB) 30. WTRU 20comprises a processor 9 configured to implement the cell reselectionmethod disclosed hereafter.

In accordance with a disclosed method, WTRU 20 performs autonomous cellreselection, with the MAC-es, used for common control channel (CCCH)data or common traffic, terminated at a Controlling RNC (CRNC).Accordingly, the MAC-es entity is associated with a common E-DCHresource set used by WTRU 20, or a common Enhanced-Radio NetworkTemporary Identifier (E-RNTI) that is selected by WTRU 20. Preferably,the MAC-es for dedicated traffic (i.e., DCCH or DTCH) is terminated in aserving RNC (SRNC) and is associated with a WTRU 20. Alternatively, theMAC-es for dedicated traffic is also terminated in the CRNC.

According to this disclosed method, WTRU 20 uses internal measurementsto make cell reselection decisions. FIG. 3 is a signal diagram of themethod that may be used by WTRU 20 when a target cell is selected in theCELL_FACH state. Upon determining to perform a cell reselection, WTRU 20ceases transmitting and receiving from source Node B 22, and clears thevariable enhanced radio network temporary identifier (E-RNTI) that wasassigned to WTRU 20 by controlling radio network controller 26.

If WTRU 20 is transmitting in the uplink (UL) when it determines that acell reselection is to be performed, then prior to moving to the targetcell, the WTRU 20 completes transmission for all hybrid access request(HARQ) processes that are active. WTRU 20 may also stop monitoring thedownlink (DL) E-DCH channels in the source cell (and, if applicable, inthe neighboring cells as well).

In an alternative, WTRU 20 may reset Transmission Sequence Numbers(TSNs) associated with the common control channel (CCCH) logical channelin the MAC-es and flush the HARQ processes if UL data is present in theHARQ processes. Alternatively, WTRU 20 can autonomously perform acomplete reset of the MAC-es/e entity (i.e., reset TSN, flush HARQprocesses, and discard any remaining segments if applicable).

WTRU 20 then receives system information blocks from target Node B 22 inthe broadcast control channel 202. If the E-DCH information element (IE)(referred to herein as E-DCH system information) is broadcasted in thetarget cell system information blocks (SIBs), WTRU 20 starts a newpreamble ramp-up phase in target cell using the broadcasted E-DCH RACHpreamble sequences. The presence of this E-DCH system informationindicates to WTRU 20 that the cell supports E-DCH in CELL_FACH. E-RACHaccess, therefore, can be achieved using a common, shared, or randomlychosen WTRU ID.

After a successful acquisition, WTRU 20 is assigned a set of E-DCHresources for use in the target cell. WTRU 20 then sends a CELL_UPDATEmessage 203 in the target cell over the assigned E-DCH resource, tocontrolling RNC 26 (CRNC). Since the MAC-es for CCCH ends in CRNC 26,the CELL_UPDATE message can be read and decoded by CRNC 26. WTRU 20 mayinclude an information element (IE) indicating the E-DCH in CELL_FACHcapability in CELL_UPDATE message 203 (e.g., IE “E-DCH in CELL_FACH”).This IE may be an enumerated IE. If the E-DCH in CELL_FACH IE ispresent, the WTRU supports E-DCH in CELL_FACH. The absence of the IEindicates that the WTRU 20 does not support E-DCH in CELL_FACH.Alternatively, the E-DCH in CELL FACH IE can be set to TRUE or FALSE toindicate whether the WTRU has E-DCH in CELL_FACH capable.

Alternatively, the E-DCH capability is implicitly detected by CRNC 26.If CRNC 26 receives the CELL_UPDATE 203 message via an E-RACH Iub frameprotocol, CRNC 26 implicitly determines that WTRU 20 supports E-DCH inCELL_FACH.

CRNC 26 may alternatively check the sub-layer from which the packet wasreceived. If delivered from the MAC-es entity, CRNC 26 determines thatWTRU 20 supports E-DCH in CELL_FACH. Otherwise, WTRU 20 does not supportE-DCH in CELL FACH.

In another alternative, source Node B 24 notifies CRNC 26 that thepacket was received over E-DCH. This alternative requires signaling overthe Iub. As such, a new Iub message can be defined or the Node B caninclude the indication with the message over the Iub frame

Accordingly, when CRNC 26 receives the CELL_UPDATE message 203 anddetermines that E-DCH in CELL_FACH is supported by WTRU 20, and alsosupported by CRNC 26, CRNC 26 allocates a new E-RNTI to WTRU 20. CRNC 26then forwards the message to Serving RNC (SRNC) 28 via radio networksubsystem application part (RNSAP) UL Signaling Transfer 205, includingthe variable E-RNTI.

Alternatively, CRNC 26 does not send an indication that WTRU 20 supportsE-DCH in CELL_FACH, but relies on SRNC 28, which is aware of the WTRU 20capabilities. As such, SRNC 28 requests CRNC 26 to allocate a “newE-RNTI” for WTRU 20, which may be accomplished, for example, via thecommon transport channel request.

SRNC 28 can tell source Node B that the E-DCH resources of the WTRU canbe released. In the case where a MAC-e is dedicated per WTRU, the MAC-eentity may be fully released. If the MAC-e entity is present per set ofE-DCH resources, the HARQ soft buffers in the source Node B are flushed,but not necessarily released. If a MAC-e/es reset is autonomouslyperformed every time a cell reselection is performed, the MAC-es entityin the SRNC, associated with the WTRU performing the reselection, alsohas to be reset.

SRNC 28, upon receiving the message from CRNC 26, generates a CELLUPDATE CONFIRM message 206, 207, including a new E-RNTI for use inCELL_FACH state. This message may be used by the network to assign E-DCHresources. Alternatively, any E-DCH resources are allocated by targetNode B 22 after a preamble power ramp-up performed when WTRU 20 needs totransmit additional traffic.

Upon receiving the CELL_UPDATE CONFIRM message 207, WTRU 20 stores thenew E-RNTI for use in future uplink access.

Alternatively, if WTRU 20 does not autonomously perform a MAC-e/es resetas disclosed above, the network may explicitly request a MAC-e/es resetto be performed by WTRU 20. The network may request a reset of theMAC-e/es entity in WTRU 20 following SRNS relocation, every time a cellreselection is performed, when a reconfiguration occurs, or when thededicated MAC-es needs to be released or reset.

In accordance with this method, a MAC-e/es reset may be performed byadding a new IE “MAC-e/es reset” indicator in the CELL_UPDATE CONFIRMmessage. Upon reception of the indicator by WTRU 20, WTRU 20 performs afull reset of the MAC-e/es (i.e., reset TSN and flush HARQ processes).If applicable, WTRU 20 may discard any remaining segments in the MAC.The MAC-e/es may also be reset by WTRU 20 autonomously resetting theMAC-e/es when WTRU 20 receives a CELL_UPDATE CONFIRM and at least one ofthe following conditions are true: a SRNS relocation has been performed,which may be detected when the IE “new U-RNTI” is present in theCELL_UPDATE CONFIRM; a reconfiguration of MAC protocol occurs within theCELL_UPDATE CONFIRM; or a reconfiguration of the radio link control(RLC) protocol occurs.

Each of the above mentioned methods of resetting the MAC-e/es may beperformed alone or in combination with one another. WTRU 20 may alsoperform a partial MAC-e/es reset, where the TSN number is set to zerofor the logical channel CCCH when the CELL_UPDATE CONFIRM message isreceived.

In accordance with an alternative method of autonomous cell reselection,the UTRAN MAC-e and MAC-es entities are both at the Node B. As a result,the E-DCH transmission for CELL_FACH WTRUs terminates at the Node B. AWTRU, therefore, is configured to make a cell reselection decision basedon internal measurements with respect to a new target cell selected aspart of cell reselection.

WTRU 20 ceases transmitting to and receiving from the source cell andstops monitoring the downlink (DL) E-DCH channels in source cell. If theWTRU is transmitting in the UL, prior to moving to the target cell, WTRU20 may complete transmission of all HARQ processes that are active atthe time it is determined that WTRU 20 is to perform a cell reselection.

WTRU 20 then performs a reset of the MAC-es/e entity (i.e., flushes HARQbuffers and resets TSN to 0 and, if applicable, may discard anyremaining segments in the MAC-e/es entity). The variable E-RNTI is thencleared by WTRU 20. The target Node B transmits SIB information in thebroadcast control channel to WTRU 20. If the E-DCH system informationIEs are broadcasted in the target cell SIBs, WTRU 20 starts a newpreamble ramp-up phase in the target cell using the E-DCH preamblesequences.

After a successful acquisition, WTRU 20 is assigned a set of E-DCHresources to use in the target cell. For collision resolution, WTRU 20may use a common E-RNTI or a random ID chosen in the MAC header. WTRU 20then sends a CELL UPDATE message including the E-DCH in CELL_FACHcapability indication, over the target cell to the controlling RNC. Thetarget cell CRNC receives the CELL_UPDATE message, and when E-DCH inCELL_FACH is supported, the CRNC allocates an E-RNTI. The CRNC thenforwards the message to the SRNC via Radio Network Subsystem ApplicationPart (RNSAP) UL Signaling Transfer including the variable E-RNTI.

Alternatively, WTRU 20 does not send E-DCH in CELL_FACH capability inthe CELL_UPDATE message. The SRNC, though, is aware of the WTRUcapability, and therefore, requests the CRNC to allocate a new E-RNTIfor WTRU 20. This may be done via the common transport channel request.The CRNC provides the new E-RNTI in the response message. Alternatively,the E-DCH capability can be implicitly detected by the CRNC. If the CRNCreceives the CELL_UPDATE via an E-RACH Iub frame protocol, the CRNC canimplicitly determine that the WTRU supports E-DCH in CELL_FACH.Optionally, the CRNC can check what sub-layer the packet was receivedfrom. If delivered from the MAC-es entity, the WTRU supports E-DCH inCELL_FACH, otherwise the WTRU does not.

Alternatively, the Node B can notify the CRNC that the packet wasreceived over E-DCH. This requires signaling over the Iub. A new Iubmessage can be defined, or alternatively, the Node B can include theindication with the message over the Iub frame.

Optionally, the SRNC tells source Node B that the E-DCH resources fromthe WTRU can be released. The source Node B can also reset the UTRANMAC-e and MAC-es and flush the HARQ soft memory buffers in the sourcecell.

SRNC preferably generates a CELL_UPDATE CONFIRM message, where itoptionally includes the “new E-RNTI” to be used in CELL_FACH state. Thenetwork may optionally use this message to assign a different set ofE-DCH resources.

Upon reception of CELL_UPDATE CONFIRM the WTRU stores the “new E-RNTI”and uses it for future uplink access.

Alternatively, the network can include a MAC-es/e reset indicator in theCELL_UPDATE_CONFIRM message.

In accordance with another alternative method for autonomous cellreselection, the E-DCH for a CELL_FACH WTRU terminates at a SRNC butCELL_UPDATE message is sent over a legacy (release 99) RACH.

The UTRAN MAC-es entity, in accordance with this method, is located atthe SRNC and associated with a particular WTRU. The WTRU is configuredto perform cell reselection using a legacy RACH procedure to transmitthe CELL_UPDATE message, instead of sending the CELL_UPDATE message overan enhanced RACH in the target cell. This allows the WTRU to undergo acell reselection without resetting the MAC-e/es entity (i.e. the WTRUdoes not need to flush the HARQ buffers and reset the TSN number to 0),allowing the incrementing of the TSN to continue after a cellreselection. Furthermore, this allows the WTRU to temporarily suspendE-DCH transmission while the cell reselection is being performed and toresume transmission after the reselection.

As disclosed above, the WTRU makes cell reselection decisions based oninternal measurements. If, as a result of the internal measurements, anew target cell is selected, the WTRU stops transmitting in a sourcecell. If data is available in the HARQ processes, the WTRU stops thetransmission of the HARQ processes such that the WTRU can continue withthe same HARQ processes in the target cell. Alternatively, the data inthe HARQ processes is flushed.

If the WTRU is transmitting in the UL, then prior to moving to thetarget cell, the WTRU may complete transmission of all HARQ processesthat are active at the time it determines that it is to perform a cellreselection.

The WTRU then ceases monitoring the DL E-DCH channels and other downlinkresources in the source cell and clears the variable E-RNTI.

The WTRU then receives the target cell PRACH information and DLinformation from the SIBs and transmits the CELL_UPDATE message. TheCELL_UPDATE message may include the E-DCH in CELL_FACH capabilityindication over the Release 99 RACH of the target cell. The CELL_UPDATEmessage is received by the CRNC and forwarded to the SRNC. The CRNCreceives the CELL_UPDATE, and allocates an E-RNTI when the WTRU is E-DCHin CELL_FACH capable. The CRNC then forwards the message, including thevariable E-RNTI, to the SRNC via RNSAP UL Signaling Transfer. In thealternative, the WTRU does not send E-DCH in CELL_FACH capability in theCELL_UPDATE. Since the SRNC is aware of the WTRU capability, it requeststhe CRNC to allocate a new E-RNTI for the WTRU, which may beaccomplished via the common transport channel request. The CRNC,therefore, provides the new E-RNTI in the response message and the SRNCincludes the E-RNTI in the CELL UPDATE CONFIRM.

The SRNC generates the CELL_UPDATE CONFIRM message, which can be used bythe network to assign dedicated E-DCH resources. Alternatively, anyE-DCH resources can be allocated by the target Node B after a preamblepower ramp-up is performed when the WTRU needs to transmit additionaltraffic. The CELL_UPDATE_CONFIRM message may include the “new E-RNTI”for the WTRU.

The SRNC then informs the source Node B that the E-DCH resources of theWTRU can be released. In the case where a MAC-e is dedicated per WTRU,the MAC-e entity is also fully released. If the MAC-e entity is presentper set of E-DCH resource, the HARQ soft buffers in the source Node Bare flushed.

Upon reception of the CELL_UPDATE_CONFIRM message, the WTRU stores thenew E-RNTI for future uplink access.

The network may explicitly request a MAC-e/es reset to be performed bythe WTRU. The network may request a reset of the MAC-e/es entity in theWTRU following the occurrence of SRNS relocation, the performance ofcell reselection, and the occurrence of a reconfiguration.

In accordance with this method, a MAC-e/es reset may be performed byadding a new IE “MAC-e/es reset” indicator in the CELL_UPDATE_CONFIRMmessage. Upon reception of the indicator by the WTRU, the WTRU performsa full reset of the MAC-e/es (i.e., reset TSN, flush HARQ processes). Ifapplicable, WTRU 20 may discard any remaining segments in the MAC. TheMAC-e/es may also be reset by sending a special reset that only flushesthe HARQ buffers without altering the TSN used for reordering. The MAC-eprotocol data units (PDUs) that are in the HARQ buffers would requireretransmission from some higher layer protocol.

Alternatively SRNC can send a special reset signal telling the WTRU torestart all currently active HARQ processes in the target cell. Thenetwork then informs the source Node B that E-DCH resources in thesource cell can be released. The network can also prepare the E-DCHresources in the target cell.

Alternatively, the WTRU can autonomously perform a reset aftercompleting transmission of all HARQ processes (using any of the resetoptions highlighted above).

In accordance with another alternative method for autonomous cellreselection, the E-DCH for CELL_FACH terminates at a Serving RNC (SRNC)for all data. In accordance with this method, the MAC-es terminates inthe SRNC for all types of data traffic, including CCCH, and theCELL_UPDATE message is received and decoded in the MAC-es in the SRNC.As disclosed in above methods, the initial steps prior to sending theCELL_UPDATE message by the WTRU are the same.

Once the CELL_UPDATE message is received by the SRNC, the SRNC sends arequest to the CRNC to allocate at least the C-RNTI; H-RNTI (if HS-DSCHin CELL_FACH/PCH is supported); E-RNTI (if E-DCH in CELL_FACH issupported); and other information provided by the CRNC in the ULsignaling transfer. The request may be signaled by introducing a newRNSAP message, or by using an existing RNSAP procedure, such as CommonTransport Channel Request.

The CRNC responds to the SRNC with the requested information, which maybe sent using the UL Signaling Transfer Indication or via the CommonTransport Channel Response message. The SRNC then transmits theCELL_UPDATE_CONFIRM message to the WTRU. The procedures following theCELL_UPDATE_CONFIRM are similar to the ones described in the methodsdisclosed above.

For all of the methods disclosed above for autonomous cell reselection,the SRNC has the option to allocate E-DCH resources to the WTRU. The setof E-DCH resources can be signaled and provided in theCELL_UPDATE_CONFIRM message, or signaled as an index to one of thebroadcasted set of resources in the SIBs using the CELL_UPDATE_CONFIRM.The WTRU may then use that set of resources to transmit uplink data inCELL_FACH.

The SRNC may chose to give these resources to the WTRU every time a cellreselection is performed, or when the WTRU has additional data to send.In accordance with the latter scenario, the WTRU may include with theCELL_UPDATE message an indication of the amount of other additional dataand the logical channel to which they belong. Alternatively, it cansignal just an indication that it has other data to transmit. The RNCcan then use this information to decide whether the WTRU requires E-DCHuplink resources. If no additional data are to be sent by the WTRU, theRNC sets up the transport channel and physical channel resources in thetarget Node B. The RNC does not signal to the WTRU any E-DCH resources.

An alternative network assisted cell reselection method is disclosedwherein the network preferably has control of cell reselection. Inaccordance with this method, the WTRU measures the channel quality andprovides these measurements to the network via measurement reports.Triggering criteria is defined for CELL_FACH WTRUs to transmit thesemeasurement reports. The network then controls cell reselection, basedon the received measurements.

The network may initiate cell reselection based on measurements takenfrom Node Bs in the alternative. If the conditions for a cellreselection are met, the network may decide to move the WTRU to CELL_DCHin the target cell, or keep the WTRU in CELL_FACH in the target cell. Inthis alternative, the WTRU is kept in CELL_FACH. The network transmitsan RRC message through a source cell to the WTRU. The WTRU, in responseto the receipt of the RRC message, stops transmitting in the source celland ceases monitoring the DL E-DCH channels in the source cell. E-DCHresources are then re-acquired in the target cell using, for example, apower ramp up procedure. The WTRU then resets the MAC es/e entitiesand/or HARQ processes by sending a MAC-es/e reset indicator in the RRCmessage and perform a full MAC-e/es reset procedure (i.e., flush HARQand reset TSN to 0).

The MAC-e/es entities may also be reset by performing a special resetthat only flushes the HARQ buffers without altering the TSN used forreordering. The MAC-e PDUs that are in the HARQ buffers would requireretransmission from some higher layer protocol. The special reset can beindicated as a special bit in one of the RRC messages ordering thehandover, or it can be specified in the cell reselection procedure forWTRUs using E-DCH in CELL_FACH. For example, the WTRU using E-DCHperforms a special reset every time a cell reselection occurs while itis in CELL_FACH or CELL_PCH. In the case where SRNS relocation occurstogether with a cell reselection, the WTRU needs to perform a fullMAC-e/es reset.

Another method for resetting the MAC entities sends a special resetsignal to the WTRU informing the WTRU to restart all active HARQprocesses in the target cell. The RNC tells the source Node B that theold connection is ending and informs the target Node B to set up for theupcoming connection.

Alternatively, the network could pre-allocate E-DCH resources in thetarget cell and provide this allocation information in the RRC message.

The source Node B, alternatively, can signal the WTRU to stoptransmission (i.e. through a zero grant). The WTRU then initiates cellreselection upon reception of this signal.

In an alternative method for mixed autonomous/network cell reselection,the WTRU decides to perform a handover to the target cell using the cellreselection criteria. Instead of acquiring the system information of thetarget cell and sending the CELL_UPDATE message to the target cell, asdisclosed above, the WTRU sends the CELL_UPDATE to the source cell withan indication of a desired target cell. The CELL_UPDATE message includesthe cell ID of the target cell.

Upon reception of CELL_UPDATE by the source Node B, the message isforwarded to the RNC. The RNC then sets up the E-DCH resources in thetarget cell, which can be resources that are specified in theCELL_UPDATE_CONFIRM or an index to one of the broadcasted set of E-DCHresources. After sending the CELL_UPDATE message, the WTRU reads theSystem Information broadcast and connects to the target cell to receivethe CELL_UPDATE_CONFIRM message. If the confirm message is not receivedby the WTRU and the CELL_UPDATE timer expires, the WTRU reattempts tosend the CELL_UPDATE over the target cell by initiating a RACH access.

The WTRU may have the option to choose to send the initial CELL_UPDATEover the source Node B or over the target Node B. The WTRU may use oneor more of the following conditions for choosing to send the CELL_UPDATEover the source Node B: the WTRU has dedicated E-DCH resources allocatedin CELL_FACH in the source Node B; the WTRU has been given RACH accessfor an UL transmission prior to initiating the cell reselection, andthus the WTRU can still use the given resources for the remaining time;the WTRU always sends CELL_UPDATE over the source cell; or the WTRU hasdata to send other than the CELL_UPDATE.

Upon reception of the CELL_UPDATE message, the SRNC has the option ofgiving the WTRU a set of resources or an index to the set of resourcesbroadcasted in the SIBs. The RNC may choose to give these resources tothe WTRU every time a cell reselection is performed, or when the WTRUhas additional data to send. The WTRU may include in the CELL_UPDATE anindication of the amount of other additional data and the logicalchannel to which they belong. Alternatively, the WTRU signals anindication that it has other data to transmit.

The RNC can then use this information to decide whether the WTRUrequires E-DCH uplink resources. If no additional data are to be sent bythe WTRU, the RNC sets up the transport channel, and physical channelresources in the target Node B but does not signal to the WTRU any E-DCHresources.

Alternatively, the network responds with CELL_UPDATE_CONFIRM (or otherRRC message) in source cell, providing the resources to use in targetcell. The WTRU may respond with a message informing network that thephysical channel reconfiguration has been completed. The network shoulduse this message as indication to stop transmitting in source cell andto release all MAC e/es resources in that cell/NodeB.

A timer is preferably started upon sending the cell update message. Ifunsuccessful or if the network responds with a R7-like CELL_UPDATECONFIRM, the WTRU should abort its attempts to communicate with thesource cell and attempt to transmit the CELL_UPDATE over the target cellusing one or a combination of the procedures described in the firstembodiment.

Although the features and elements are described in particularcombinations, each feature or element can be used alone without theother features and elements or in various combinations with or withoutother features and elements. The methods or flow charts provided may beimplemented in a computer program, software, or firmware tangiblyembodied in a computer-readable storage medium for execution by ageneral purpose computer or a processor. Examples of computer-readablestorage mediums include a read only memory (ROM), a random access memory(RAM), a register, cache memory, semiconductor memory devices, magneticmedia such as internal hard disks and removable disks, magneto-opticalmedia, and optical media such as CD-ROM disks, and digital versatiledisks (DVDs).

Suitable processors include, by way of example, a general purposeprocessor, a special purpose processor, a conventional processor, adigital signal processor (DSP), a plurality of microprocessors, one ormore microprocessors in association with a DSP core, a controller, amicrocontroller, Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs) circuits, any other type of integratedcircuit (IC), and/or a state machine.

A processor in association with software may be used to implement aradio frequency transceiver for use in a wireless transmit receive unit(WTRU), user equipment (UE), terminal, base station, radio networkcontroller (RNC), or any host computer. The WTRU may be used inconjunction with modules, implemented in hardware and/or software, suchas a camera, a video camera module, a videophone, a speakerphone, avibration device, a speaker, a microphone, a television transceiver, ahands free headset, a keyboard, a Bluetooth® module, a frequencymodulated (FM) radio unit, a liquid crystal display (LCD) display unit,an organic light-emitting diode (OLED) display unit, a digital musicplayer, a media player, a video game player module, an Internet browser,and/or any wireless local area network (WLAN) module.

What is claimed is:
 1. A method performed by a wireless transmit/receiveunit (WTRU), the method comprising: communicating with a source cell;performing measurements on a plurality of neighboring cells;transmitting, via the source cell, information from the measurements;receiving, via the source cell, a message indicating that the WTRU is tocommunicate via a target cell, the target cell being one of theplurality of neighboring cells; and after receiving the message: ceasingusing an enhanced radio network terminal identifier (E-RNTI) associatedwith the source cell; resetting a medium access control; receiving, viathe source cell, an E-RNTI associated with the target cell; andcommunicating, via the target cell, using the received E-RNTI.
 2. Themethod of claim 1, further comprising receiving resource allocationinformation via the target cell.
 3. The method of claim 2, wherein thecommunicating via the target cell includes communicating over anenhanced dedicated channel in accordance with the resource allocationinformation.
 4. The method of claim 1, further comprising: initiating atimer in response to the received message.
 5. The method of claim 1,wherein the received message is a handover message.
 6. The method ofclaim 1, wherein the received message includes an identification of thetarget cell.
 7. The method of claim 1, wherein the resetting of themedium access control includes flushing a hybrid automaticretransmission request buffer in the WTRU.
 8. The method of claim 1,wherein the resetting of the medium access control includes resetting atransmission sequence number in the WTRU.
 9. The method of claim 1,further comprising: transmitting, in response to the received message, arandom access preamble via the target cell.
 10. The method of claim 1,wherein the received E-RNTI is assigned to the WTRU by a radio networkcontroller.
 11. A wireless transmit/receive unit (WTRU) comprising: atransmitter; a receiver; and a processor to: perform measurements by thereceiver on signals received from a plurality of neighboring cells;cause the transmitter to transmit, via a source cell, information fromthe measurements; process a message received by the receiver via thesource cell, the message indicating that the WTRU is to communicate viaa target cell, the target cell being one of the plurality of neighboringcells; and after processing the message, reset a medium access control,cease using an E-RNTI associated with the source cell, and cause theWTRU to communicate, via the target cell, using an enhanced radionetwork temporary identifier (E-RNTI) associated with the target celland received by the receiver via the source cell.
 12. The WTRU of claim11, wherein the processor processes resource allocation informationreceived by the receiver via the target cell.
 13. The WTRU of claim 12,wherein the WTRU communicates, via the target cell, over an enhanceddedicated channel in accordance with the resource allocationinformation.
 14. The WTRU of claim 11, wherein the processor initiates atimer in response to the message received by the receiver.
 15. The WTRUof claim 11, wherein the message received by the receiver is a handovermessage.
 16. The WTRU of claim 11, wherein the message received by thereceiver includes an identification of the target cell.
 17. The WTRU ofclaim 11, wherein the resetting of the medium access control by theprocessor includes the processor flushing a hybrid automaticretransmission request buffer in the WTRU.
 18. The WTRU of claim 17,wherein resetting of the medium access control by the processor includesthe processor resetting a transmission sequence number in the WTRU. 19.The WTRU of claim 11, wherein the received E-RNTI is assigned to theWTRU by a radio network controller.
 20. The WTRU of claim 11, whereinthe processor, in response to the message received by the receiver,causes the transmitter to transmit a random access preamble via thetarget cell.